by Jack Norris, RD • Last updated July 2013
- Serum Cadmium Levels
- Cadmium and Vegan Diets
- Cadmium and Zinc
- Zinc and Alzheimer’s Disease
- A Modest Zinc Supplement
- Zinc Supplements and Cadmium Contamination
Cadmium is a toxic heavy metal associated with osteoporosis, liver, and kidney disease. The only study measuring cadmium levels in vegans is from the Slovak Republic and showed vegans to have higher levels than omnivores, and levels high enough in some vegans to raise the concern of the researchers. Correcting iron deficiency can prevent the body from absorbing cadmium. A modest zinc supplement is probably safe and could also reduce cadmium absorption as well as neutralize cadmium’s harmful effects in the body.
Related link: Zinc.
Background on Cadmium
Cadmium is a common heavy metal found in soil due to the use of phosphate fertilizers (1). Cadmium has a long biological half-life and it can take 30 years for the body to rid itself of it (2). Cadmium toxicity can cause painful osteoporosis, a reduction in kidney function (2), and liver toxicity (1). Smokers tend to have higher cadmium levels due to high levels in tobacco.
|Table 1. Serum Cadmium Levels|
|Study from Slovak Republic1|
|To convert blood cadmium from µg/l to nmol/l, multiply by 8.897. To convert urine cadmium from µg/g creatinine to nmol/mmol creatinine, multiply by 1.006. (3)|
Cadmium and Vegan Diets
A 2006 study from the Slovak Republic, of non-smokers, found vegans to have significantly higher blood levels of cadmium than non-vegans (1). See Table 1 for the amounts for each diet group. However, both groups were relatively low compared to the reference range. Duration of vegetarianism was a risk factor for cadmium concentration above 5 µg/l a level the researchers believed represented health risks. Six (8%) vegetarians had levels above 5 µg/l and their average duration of vegetarianism was 11 years vs. 6 years in those with levels lower than 5 µg/l.
Cadmium concentrations were also correlated with whole grain intake (the cadmium is stored in the bran) and seeds, but not legumes. The researchers didn’t measure for damage from cadmium toxicity and believed that the higher antioxidant status of the vegetarians could protect them from the oxidative damage of cadmium (which may not be the only type of damage).
To get an idea of cadmium levels in toxicity cases, a case study of a non-smoking man who presented with severe osteoporosis and impaired kidney function had a cadmium blood level of 420 µg/l (2). It was determined that he had been exposed to cadmium in his occupation as a silver jeweler.
Blood cadmium levels as low as .5 to .8 µg/l have been associated with chronic disease. A 2012 report of 8,989 adults from the United States National Health and Nutrition Examination Survey (NHANES) 1999–2004 found such levels to be associated with all-cause mortality (1.50, 1.07-2.10), cardiovascular disease (1.69, 1.03-2.77), and heart disease (1.98, 1.11-3.54) after a follow-up of 4.8 years and many adjustments for potential confounding variables (3).
The amount of cadmium in food is dependent on the amount of cadmium in the soil. It is generally thought that undeveloped and developing countries have more cadmium in their soil. Additionally, the Slovak Republic appears to have higher cadmium levels than most of the countries in Europe (12). There have also been attempts to reduce the amount of cadmium in the environment in the United States by containing hazardous waste (4).
Cadmium and Zinc
So what’s a vegan to do? Is this something that we should worry about?
In a 2012 article in the scientific journal Medical Hypotheses, Mark McCarty writes about the dangers of cadmium and suggests supplementing with minerals, especially zinc (5), to counter cadmium damage.
For some background, note that the RDA for zinc is 11 mg for men and 8 mg for women, and vegans in EPIC-Oxford had average intakes of 7.9 mg for men and 7.2 for women (not including supplements).
McCarty goes through the research concluding that a high-normal level of cadmium in the blood is linked to vascular disease, diabetes, and cancer. He points out that shellfish and organ meats (liver and kidney) tend to be relatively high in cadmium, but otherwise the chief dietary sources are green leafy vegetables, whole grains, tubers, and root vegetables.
McCarty says that cadmium is absorbed using the same mechanisms for absorbing iron which are up-regulated in iron deficiency and therefore iron deficiency can increase cadmium absorption. I have previously written about how this can also be the case for manganese (see Manganese). Based mostly on animal studies, McCarty argues that the cadmium would also compete with zinc and calcium absorption mechanisms and, therefore, consistent supplementation plans of both minerals “confer significant protection from cadmium-mediated pathology.”
McCarty makes another argument in favor of zinc supplementation: “It appears that cadmium is innocuous as long as it remains bound to [the mineral-binding protein] metallothionein. This finding is particularly intriguing in light of the fact that cellular expression of metallothionein tends to increase with increased zinc exposure.” Intakes of supplemental zinc in the range of 15–50 mg daily have been shown to boost metallothionein in healthy young men.
McCarty then mentions a number of studies in humans suggesting zinc can be protective:
- A study found that the correlation between serum cadmium and carotid artery thickness was pronounced in subjects with serum zinc in the lower two tertiles but not in the upper tertile.
- A study on urinary cadmium and prostate-specific antigen (PSA) observed a significant correlation in men whose daily zinc intake was below the median level of 12.7 mg.
- A case-control study linked prolonged zinc supplementation with reduced risk for breast cancer in both pre-menopausal and post-menopausal women.
- A clinical trial showed that 45 mg zinc per day exerts a systemic anti-inflammatory effect.
- Supplemental zinc can improve immune function in the elderly.
The only harm from zinc supplementation that McCarty mentions is decreasing copper absorption, something that might not be such a concern for vegans. The one reliable study looking at vegan copper intakes, from the USA, found vegans to have an intake of 3.1 mg (men) and 2.2 mg (women) versus 1.3 (men) and 1.5 (women) for non-vegetarians. The RDA for copper is .9 mg for both men and women (6).
One concern about zinc supplementation is whether it could lead to Alzheimer’s Disease (AD). There is some evidence that iron, copper, and zinc are concentrated in the plaques found in the brains of people with AD. Whether copper might be the cause of AD has been of some debate, but there appears to be few if any published studies implicating zinc intakes with AD; in fact, more the opposite appears to be true.
Zinc and Alzheimer’s Disease
First of all, it’s not clear that any metals are the cause of Alzheimer’s Disease. A 2011 meta-analysis performed by researchers from Loma Linda University and George Mason University disputes the idea that metals cause Alzheimer’s (7). They conclude that the findings of higher mineral levels in the brain, especially iron, have suffered from citation bias, whereas many of the findings in the literature have shown no increased levels of such metals. They conclude:
Using meta-analysis and systemic review methodologies we have identified the wide-spread misconception in AD literature that iron, and to a lesser degree zinc and copper, levels are increased in AD brain….In light of our findings it will be important to re-evaluate the brain metal-overload hypothesis in AD and critically review related research and review articles.
But even if levels of zinc are increased in the brain of people with AD, it does not mean that higher zinc intakes are the cause.
A 1998 cross-sectional study found that zinc levels in cerebrospinal fluid (CSF) were significantly lower in AD patients than in controls (.12 vs. .22 mg/l) and that serum zinc levels were the same (8). The authors stated, “There was no significant correlation in AD patients between the CSF or serum levels of iron, copper, zinc, and manganese and the following values: age, age at onset of AD, duration of AD, and scores of the Minimental State Examination.”
A 2010 cross-sectional study found blood zinc levels to be significantly lower in AD patients compared to controls (9).
A 2006 prospective study found that after 6 years of follow-up, dietary intakes of copper, zinc, and iron were not associated with cognitive decline. However, among the people who consumed a diet high in saturated and trans fats, a faster decline was seen with higher copper (but not zinc or iron) consumption (14).
Finally, a 2012 study gave patients with mild to moderate AD 150 mg of slow-releasing zinc per day for 6 months (11). The zinc supplementation stabilized the AD symptoms, while the placebo group got worse. It wasn’t clear if this study was double-blinded and it could simply be that subjects with mild to moderate AD symptoms had low zinc intakes due to a poor diet, which is not uncommon. However, the researchers believed that it was due to zinc reducing copper levels. Although only one patient had to have their zinc dose lowered because copper decreased to deficiency levels, the researchers stated that anyone taking zinc supplements to stabilize AD symptoms should only do so under a doctor’s supervision due to potential copper deficiency.
The Linus Pauling Institute says that while copper deficiency is uncommon, one of the signs is an anemia that is unresponsive to iron therapy but corrected by copper supplementation (link).
In conclusion, it seems very unlikely that a modest zinc supplement could contribute to AD.
A Modest Zinc Supplement
There is evidence that zinc supplements up to 50 mg per day should be safe. But that is a lot of zinc and might not be worth any risks when the science on zinc supplementation and chronic disease is far from conclusive. To err on the side of safety, I would consider a “modest” supplement to be 10 to 25 mg per day. Based on the evidence above, that should be safe and will hopefully provide some protection against any cadmium or copper toxicity.
Zinc Supplements and Cadmium Contamination
In 2001, Krone et al tested six zinc supplements from Seattle area health food stores to see if they contained cadmium (13). According to the authors, “Because the chemical properties of [zinc] and cadmium (Cd) are so similar, these two elements invariably occur together in nature.”
They found that the single zinc supplements had very low levels of cadmium whereas the multi-mineral supplements had enough that taking the RDA of zinc would provide up to 2 µg of cadmium (20% of the daily limit recommended by the US Federal Drug Administration (FDA)). It also happens that three of the supplements with low levels of cadmium were in the form of zinc gluconate whereas none of the multi-mineral preparations were the gluconate form. So, it could be that zinc gluconate is unlikely to have much cadmium or that single zinc supplements are unlikely.
According to their website, the supplement manufacturer Kirkman, from Oregon, does a rigorous job testing their supplements for contamination of cadmium and other impurities (more info). They also have an article on their site, Cadmium: A Serious Heavy Metal and Topic. They ship outside the United States.
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